The present invention relates to a cavity filter tuning screw and more particularly to a tuning screw for tuning the frequency relationship of the filter and/or the coupling coefficient factor between center conductors in the filter. The invention also relates to a tuning arrangement that includes such a screw that can be placed, for instance, in the center conductor of the cavity filter or its lid, and also at some other suitable position in the chassis of the cavity filter.
Radio base filters can be constructed and manufactured as so-called cavity filters which consist of a plurality of cavities either with a separate center conductor in each cavity or more than one center conductor per cavity. These filters are used, for instance, in base stations for GSM-based mobile telephony.
The filters are generally constructed with the aid of different components, such as a chassis or lid, for instance. However, filters that are not tuned when manufactured must be adjusted subsequent to being fitted, because of the influence of tolerances on the filter components. This adjustment of the filter properties is effected by tuning the filter with respect to its resonator frequencies and the couplings between the center conductors in the filter.
In a cavity filter, each cavity that includes a center conductor functions as an electric oscillator circuit that can be represented by a parallel oscillator circuit that includes an inductive part L and a capacitive part C when the filter is tuned on a quarter wavelength of the received signal. The resonator frequencies of the filter are therewith determined through the inductive and capacitive parts.
The frequencies of the cavity filter are tuned by changing the length of the center conductor and therewith the inductance, and by changing the distance from the center conductor in the cavity to the cavity lid and also to the cavity side walls, which results in a change in capacitance. Frequency adjustments must be made very accurately and also in a manner which will ensure that the set frequencies are maintained.
Tuning of the coupling coefficient factor between the center conductors in the cavity filter is effected by changing their relative distances or by changing the geometry of the filter in the space between the center conductors. This latter measure may entail providing the cavity partition walls with appropriate apertures for instance and/or providing a tuning device between said center conductors.
FIG. 1 illustrates a known method of tuning a cavity filter. The Figure is a sectioned view of a cavity filter 10 which is defined by a cavity bottom 12 and side walls 13. The cavity has provided therein a center conductor 11 which in the illustrated case is formed integrally with the bottom 12 of the cavity and extends out therefrom. The whole of the cavity is covered by a lid or cover member 15. Tuning is effected with the aid of a tuning plate 14 attached between the cavity walls and lid and functioning as an adjustable earth plane. The tuning plate is flexible to some extent, so that its position relative to the upper edge of the center conductor can be changed by screwing a screw 17 through the cavity lid 15 until the correct distance is achieved between tuning plate 14 and center conductor 11. Alternatively, tuning can be effected solely with the aid of the screw 17, the position of which in the filter lid is fixed by means of a counter nut or lock nut.
0 525 378 A2 teaches a self-locking tuning screw which consists of a first part 1 that includes two threaded segments 4, 5 and an intermediate plain or non-threaded segment 3, and a non-threaded or plain second part 2 intended for insertion in a microwaveguide. The first part of the screw includes a screwdriver accommodating recess that includes several edges which form slots at the non-threaded segment 3.
The present invention relates to a component tuning arrangement, preferably for tuning a cavity filter.
One object of the invention is to provide a screw that can be used in a cavity filter to tune the frequency relationship of the filter and which will be self-locking when fitted in the center conductor, the wall, or the lid of the filter.
Another object of the invention is to provide a screw which can be used in a cavity filter to tune the coupling coefficient factor between two center conductors in the filter and which will be self-locking when fitted in an appropriate position in the bottom of the cavity and its side-walls or lid respectively.
Still another object of the invention is to provide a tuning device which consists solely of one component in the form of a self-locking screw, thereby eliminating screw play.
These objects are achieved in accordance with the invention by means of a screw which has at least one slot extending in its axial direction and which is configured with at least one thicker part. As a result of this thicker part, the screw slot will be squeezed together as the screw is screwed-in, thereby exerting on the walls of the screw hole a force which ensures that the screw will be firmly locked in the position desired.
The inventive arrangement for tuning the frequency relationship of the filter is achieved by mounting the screw in a center conductor, side wall or lid. Tuning of the coupling coefficient factor between two center conductors is achieved by placing the screw between the center conductors in the bottom of the filter, the lid or its side walls. Tuning is effected by appropriate adjustment to the position of the screw head.
More specifically, the inventive arrangement is characterised by the features set forth in the accompanying claims.
One advantage afforded by the inventive tuning arrangement is that it consists of only one component. Because the tuning screw included in the arrangement is self-locking with a constant moment of force along the full length of the screw, no counter nuts or lock nuts are required to lock the screw in its set, desired position. Neither is a tuning plate required in order to tune the filter. This results in more rational and less expensive manufacture.
Another advantage is that tuning is facilitated by virtue of the fact that the inventive tuning arrangement includes two screwdriver accommodating recess, of which one is intended for turning the screw and the other for adjusting the position of the screw subsequent to having placed the lid in position.
Another advantage resides in greater flexibility, since the tuning screw can be readily replaced with screws of other designs, for instance with respect to the head of the screw.